Wednesday 11 June 2014 07.20 EDT
First published on Wednesday 11 June 2014 07.20 EDT

Comparing short-form improv, those knockabout games featured on Whose Line is It Anyway?, with the more naturalistic, long-form tradition favoured by graduates of Chicago's Second City theatre group is like comparing "Louie Louie with Thelonius Monk," says Michael McCarthy.

One of the founders of the Cat Laughs comedy festival in Kilkenny 20 years ago, the former Saturday Night Live writer always wanted the festival's lineup to feature the "accidental heart" of improv as well as slick international standup sets. In its first year, he brought over a Second City troupe fronted by Cheers star George Wendt and he returned in 1996 with a group that included Bill Murray and Murray's brothers Brian and Joel.

This month, McCarthy joined Wendt and Joel in a Second City quintet that also included Bernadette Birkett and current SNL writer Katie Rich, eager to take part in an All-Star International Improv nights. But there was a problem: the group weren't enamoured with the short-form style favoured by the Irish troupe, led by Ian Coppinger, and the UK contingent, led by Whose Line? regular Stephen Frost, one of those who are bringing a live version of the much-loved Channel Four show to the Edinburgh fringe this summer. And the Americans rather suffered by comparison with the "home" team during the opening-night anniversary gala.

Mutual respect endures. But as Wendt, a guest performer on Whose Line? when it aired in the 90s, admits: "We saw the Irish and English guys, and they did a tremendous set of high-energy games, very funny, and we were a little intimidated. I thought, 'Gosh, I just don't have that kind of energy any more and frankly, I'm not that quick.' I'm sure Katie, Bernadette and Joel [who tours the US with Whose Line? regulars Greg Proops, Ryan Stiles and Colin Mochrie] could play with those guys. But Joel came up with the idea of going the other way, to do something long-form and slow."

So when the Americans took to the stage in their own showcase, they devoted most of their hour to a game called Armando Diaz, in which the players take it in turns to step out of a Greek-style chorus and deliver a monologue based on audience suggestions, with the rest of the group then improvising scenes inspired by it. The laughs were less rapid but much more explosive, and there was far greater development of character. And while I didn't think for a second that any of it was scripted (as some watching apparently did), I was staggered to learn that the five had never previously performed together. Wendt and Birkett are married, sure enough. But McCarthy just rounded up whoever could make the trip.

Indeed, so second nature are the Second City rules of supporting and not outshining your fellow improvisers – affirming what they say then adding to it, making statements rather than asking questions, rejecting jokes in favour of establishing a character's reality – that Wendt and his colleagues just relied upon their training, confident that their subconscious would organically present funny scenes with human truth.

John Candy, left, and Bill Murray at the Second City in Chicago in 1973

McCarthy likens Second City to a "cult" that its students, who perform upwards of 300 shows a year, "focus on with white heat". And impressive as the alumni are – including the likes of John Belushi, Dan Aykroyd, Tina Fey, Joan Rivers, Steve Carrell, Mike Myers, John Candy, Amy Poehler, Fred Willard and even David Mamet (who scrubbed dishes at the Chicago school) – its legacy is still very much evident, with Stephen Colbert taking the US late-night chat-show crown from David Letterman and SNL once again dominated by Second City grads.

Inspired by Viola Spolin's 1963 book, Improvisation for the Theater, and on the back of the success of Mike Nichols and Elaine May with Chicago's Compass Players, Second City forged its identity under eccentric teachers such as Del Close (whose battered jacket inspired the look of Belushi and Aykroyd's Blues Brothers) and producers such as David Shepherd, whom Birkett credits with striving to make it "a theatre for the people".

"No one's trying to beat anyone else, no one's trying to be funnier. It's this evolving theatre company that captures the zeitgeist," she says. "You're listening to what the audience is saying, so it's social satire in a really mature form. You're tapping into their consciousness."

Rich joined SNL in November and still teaches improv. But her Second City education informs much more than her career. "It's in everything you do, your whole life," she says. "If you're depressed or stuck on something, the way to get out is to focus on another person – get out of your own head and think about how to make them look good." Still, for all this emphasis on ensemble and even "dysfunctional family", some Second City stars always shone brighter than others. "Chris Farley was the one during my time there," recalls Joel Murray. "You just saw him coming like a comet. And John Belushi, you could never take your eyes off him."

Much more instructive regarding Second City's influence, perhaps, is what he reveals of his brother, one of the most popular but enigmatic comedy performers alive. "Bill told me, 'I talk about myself enough, so when I'm out somewhere and I meet somebody, I'll ask them repeatedly about what they do and their career. I don't want to talk about Stripes or what fun it was to do Meatballs. But I'll talk to a guy who's an estimator, a home appraiser or something like that, and I'll be fascinated by what they do. Suddenly, five years later, I'm playing a home appraiser in a role and thinking, "I kind of know who this guy is.'"

Joel, who has a role on the TV show Mad Men, relates: "I've always tried to that a little bit, too, finding myself in an audition for an ad man thinking, 'Yeah, I can do this.'"

Wendt carried the Second City messages of "keeping it fun" and "always respecting the audience" to his defining role as barfly Norm Peterson in Cheers. But he acknowledges that improv is, by its nature, ephemeral: "There's been so much brilliance over the years, and a lot of rubbish, mind you, that's disappeared into the ether. Hopefully though, it's resurfaced in the unconscious of all those audiences who saw it or the cast who played it. And likely lots of it got filtered down to a piece in a script somewhere."